1. Field of the Invention
The present invention relates to high-frequency oscillators provided in various high-frequency apparatuses, and more particularly, the present invention relates to a frequency-selective high-frequency oscillator that switches operation between two different oscillation frequencies.
2. Description of the Related Art
Some known high-frequency oscillators, such as voltage controlled oscillators, function as frequency-selective high-frequency oscillators that oscillate high-frequency signals with two different frequencies as a single unit (circuit). These frequency-selective high-frequency oscillators each include first and second resonant circuits that oscillate at different frequencies from each other; first and second amplifying circuits that are connected to the first and second resonant circuits, respectively; a buffer circuit that amplifies and outputs an output signal from the first or second amplifying circuit; and a selection circuit that selects the first or second amplifying circuit. This selection circuit includes first and second switching circuits that are connected to the first and second amplifying circuits, respectively. Furthermore, first and second control signal input terminals for independently operating the respective first and second switching circuits are also provided. In such a frequency-selective high-frequency oscillator with the above-described structure, a control signal is input to either of the first and second switching circuits by selecting the first or second control signal input terminal according to the desired frequency to be output. The amplifying circuit to be operated is thus selected to oscillate and output a signal with the desired frequency (for example, refer to Japanese Unexamined Patent Application Publication No. 2001-237640).
Another example of these frequency-selective high-frequency oscillators is shown in FIG. 12.
FIG. 12 is a circuit diagram of another related frequency-selective high-frequency oscillator, which the applicants of the present application are unable to confirm whether it has been publicly disclosed or used so as to constitute prior art. In the following description, the structure of the frequency-selective high-frequency oscillator is outlined by describing the flow of a high-frequency signal. A description of the passive elements is not provided.
Referring to FIG. 12, a first resonant circuit 101a with a first resonant frequency f1 is connected to the base terminal of an NPN transistor Tr102 of a first amplifying circuit 103a, and a second resonant circuit 101b with a second resonant frequency f2 is connected to the base terminal of an NPN transistor Tr103 of a second amplifying circuit 103b. The first amplifying circuit 103a oscillates at the resonant frequency f, based on the first resonant circuit 101a, that is, in conjunction with the first resonant circuit 101a, and outputs part of the high-frequency signal. Similarly, the second amplifying circuit 103b oscillates at the resonant frequency f2 based on the second resonant circuit 101b, that is, in conjunction with the second resonant circuit 101b, and outputs part of the high-frequency signal. A buffer circuit 104 including an NPN transistor Tr101 is connected to the first amplifying circuit 103a including the transistor Tr102 and to the second amplifying circuit 103b including the transistor Tr103. The buffer circuit 104 receives a high-frequency signal output from either of these amplifying circuits, and then amplifies and outputs the high-frequency signal to an output terminal 106.
Furthermore, this high-frequency oscillator includes a first switching circuit (SW1) 102a including a PNP transistor Tr104 having the collector terminal connected to the base terminal of the transistor Tr102 of the first amplifying circuit 103a and having the emitter terminal connected to a power terminal 105; and a second switching circuit (SW2) 102b including an NPN transistor Tr105 having the collector terminal connected to the emitter terminal of the transistor Tr103 of the second amplifying circuit 103b and having the emitter terminal grounded. The base terminals of the transistors Tr104 and Tr105 of these switching circuits 102a and 102b are connected to a terminal 107 for inputting a switching control signal.
With the structure described above, when the switching control signal is “Low”, the first switching circuit 102a turns ON and the second switching circuit 102b turns OFF, and hence only the first amplifying circuit 103a operates. As a result, a high-frequency signal with the first resonant frequency f1 is oscillated and output. In contrast, when the switching control signal is “High”, the first switching circuit 102a turns OFF and the second switching circuit 102b turns ON, and hence only the second amplifying circuit 103b operates. As a result, a high-frequency signal with the second resonant frequency f2 is oscillated and output.
In the frequency-selective high-frequency oscillator disclosed in the Japanese Unexamined Patent Application Publication No. 2001-237640, however, control is complicated because two different control signals are necessary to select a frequency.
In contrast, in the frequency-selective high-frequency oscillator shown in FIG. 12, only one control signal is sufficient. However, this causes another problem as described below.
In the high-frequency oscillator shown in FIG. 12, when the switching control signal becomes “High”, the second switching circuit 102b turns ON. In this case, if noise is input from the switching control signal input terminal 107 along with the switching control signal, the noise is input to the transistor Tr103 of the second amplifying circuit 103b via the second switching circuit 102b and is superposed on the oscillated signal. In this manner, the high frequency signal having noise superposed thereon is input to the transistor Tr101 of the buffer circuit 104 and amplified, and therefore noise is also amplified. This causes the S/N ratio of the output high-frequency signal to decrease.